The macula of the human eye is generally understood as having been designed for providing detailed vision. The macula can have a relatively large area, measuring often about six millimeters in diameter and covering about 21.5 degrees of visual angle centered on the fovea. The macula is understood as being responsible for producing central, high resolution, color vision, and, as such, any damage to the macula (e.g., damage caused by macular degeneration) can result in impairment or loss of such vision.
The human macula can be divided into a number of sub-regions, namely the umbo, foveola, foveal avascular zone, fovea, parafovea, and perifovea areas. The fovea comprises a small area dominated by cone-shaped cells, and is surrounded by parafovea, which is sub-region of the macula, generally dominated by rod-shaped cells. As detailed in U.S. Pat. No. 9,504,379, the entire teachings of which is incorporated by reference herein, the rod-shaped cells appear to be responsible for vision in dim light, while the cone-shaped cells are understood to be responsive to bright light and colors. In young adults, the number of rod-shaped cells outnumbers the cone-shaped cells by approximately 9:1. This proportion of the rod-shaped cells to cone-shaped cells changes as a person ages.
The health and function of the rod-shaped and the cone-shaped photoreceptors are maintained by the pigmented layer of retina or retinal pigment epithelium (RPE), the Bruch's membrane (BM), which is located between the retinal pigment epithelium and the fenestrated choroidal capillaries of the eye, and the capillary lamina of choroid or choriocapillaris, which is located adjacent to Bruch's membrane in the choroid (collectively referred to as the RPE/Bruch's membrane complex).
The RPE is a dedicated layer of nurse cells behind the neural retina, which is understood to be responsible for sustaining photoreceptor health in a number of ways, including, but not limited to, maintaining proper ionic balance, transporting and filtering nutrients, providing retinoid intermediates to replenish photopigment bleached by light exposure and absorbing stray photons. Bruch's membrane, which comprises a vessel wall of about 2-6 μm, further separates the RPE and the choriocapillaris. The choriocapillaris provides blood flow to the outer retina, particularly the rods.
An impairment of the RPE/Bruch's membrane complex can result in reduced transportation of nutrient and oxygen to the photoreceptors and reduced clearance of by-products of bleaching, such as opsin, thereby impairing the health and function of the photoreceptors. This can be especially true with the rod-shaped cell photoreceptors, which are responsible for scotopic, or dark-adapted vision.